This invention relates to printed circuit boards with busbar interconnections for carrying large currents between circuit elements mounted on the circuit board.
Busbars are utilized for making large current interconnections between circuit elements mounted on printed circuit boards. FIG. 1 shows a conventional printed circuit board utilizing busbars as large current interconnections, which is disclosed, for example, in Japanese Utility Model Application Laid-Open (Kokai) 55-77882.
In FIG. 1, busbars 1, made of slender elongated copper plates 12 mm wide and 1.2 mm thick for example, have bar ring portions 2 at respective ends thereof, each of which bar rings has a through hole 5 through which leads 4 of a circuit element 3 extends. The bar rings 2 are inserted into respective through holes formed in the board 6 of an electrically insulating material; the bar rings 2 have a height substantially equal to the thickness of the board 6 so that the lower ends of the bar rings 2 are flush, when inserted, with the lower surface of the board 6 on which printed circuit patterns 10 made of copper foil are formed. The busbars 1 are soldered to the circuit board 6 via copper foil patterns 8 formed on the board 6. The leads 4 of circuit elements 3 are inserted into the through holes 5 in the bar rings 2 and are soldered thereto by means of solder 9. Thus, the leads 4 of the elements 3 are directly soldered to the bar rings 2 of the busbars 1, so that the electrical impedances therebetween can be reduced.
However, the method of mounting the busbars and making interconnections as shown in FIG. 1 has the following disadvantage.
Namely, the bar rings 2 of the busbars 1 projecting from the busbars 1 are tapered toward their ends so as to enhance the solderability thereof, the soldering being effected at the ends of the bar rings 2; thus the electrical connection between the busbars 1 and the solder 9 is secured only at the ends of the bar rings, so that the reliability of the electrical connection between the busbars 1 and the leads 4 is low, and the impedance therebetween cannot be reduced below a certain limit. In addition, the connections between the busbars 1 and the leads 4 of the circuit elements 3 are mechanically weak and are not suited for passing large currents therethrough. Further, the busbars 1 are mounted to the printed circuit board 6 via copper foil patterns 8, which are formed on the board 6 for the sole purpose of securing the busbars 1 to the board 6; this, however, makes the production steps complicated and time-consuming, and thereby increases the production cost.
Thus, provided herein is a mounting and connecting structure of the busbars as shown in FIG. 2.
In FIG. 2, the busbars 1 have bar rings 21, which fit into the respective through holes 7 formed in the circuit board 6. The height of the bar rings 21 is equal to the thickness of the board 6 such that the upper ends of the bar rings 7 are flush with the upper surface of the board 6. Each lead 4 of circuit elements 3 has a base portion 41 of a large diameter and a pin portion 42 extending therefrom which has a diameter and a height slightly smaller (e.g., by 0.2 mm to 0.5 mm) than the inner diameter and the height, respectively, of the bar rings 21. The assembling of these parts is effected as follows: the bar rings 21 of the busbars 1 are inserted into respective through holes 7 of the board 6 from below, and then the pin portions 42 of the leads 4 are inserted from above into respective through holes in the bar rings 21; thereafter, the lower side of the board 6 to which the busbars 1 are attached is dipped into a bath of molten solder, so that the molten solder enter the gap between the bar rings 21 and the pin portions 42 of the leads 4 by means of the capillary action. Thus, the circuit elements 3 are fixedly secured to the board 6, the pin portions 42 of the leads 4 being soldered to the bar rings 21 of the busbars 1 via the solder 91.
The mounting and connecting method of the busbars as shown in FIG. 2, however, still suffer from the following disadvantages. Namely, according to the above structure, the dipping of the board into molten solder bath is necessary for securing connection between the busbars 1 and the leads 4; this increases the number of assembling steps. Further, since the temperatures of the board and the busbars rise during the immersion thereof into the molten solder bath, strains may be generated after they are cooled down, due to the difference in the coefficients of expansion of the busbars 1 and the board 6. Furthermore, the formation of bar rings 21 on the busbars 1 is a complicated and time-consuming work process, which requires a special tool for working. In addition, the printed circuit boards as produced above have the problem that, since the busbars are in close contact with the board 6, heat dissipation from the large current carrying busbars 1 can be effected only inefficiently, and hence the amount of current that can be passed through the busbars is thereby limited.
The conventional busbar structure has still another kind of problem; let us describe this problem by referring to FIG. 3, which shows a plan view of a printed circuit board which is disclosed, for example, in Japanese Utility Model Laid-Open (Kokai) 60-151186.
In FIG. 3, the circuit elements 3a through 3d are mounted on the back surface of the electrically insulating board 6, to be connected via the busbars 1 through the through holes 7 formed in the board 6. The busbar interconnection structure of FIG. 3 has the advantage that each busbar can be punched out from a single metal plate even when the circuit interconnection pattern thereof is complicated. However, when, for example, a terminal of a circuit element 3a, whose terminals are connected to those of the neighboring element 3b, is to be connected to a terminal of another element 3d, a long detour 1a (shown by the hatches in the figure) must be formed in the busbar 1 which connects them, so that crossing of the busbars can be avoided. This makes the busbar circuit pattern complicated, and a wide area is necessary on the board for carrying the busbars thereon; as a result, the overall size of the circuit board becomes larger, and the mounting density of the circuit elements on the board is thus reduced. In addition, the punching of the complicated forms of the busbars generates much metal remains which are left in the metal plate after the busbars are cut out.